In the molding of articles of plastic and other like materials, as in injection molding, blow molding, rotational molding, compression molding, reaction injection molding and other processes, efficient operation of the molding process depends upon effective control of thermal conditions in the mold. Thus, in many plastic molding processes relatively rapid cooling of the mold to a temperature at which the molded article can be removed without distortion is a critical part of the process. In other plastic molding procedures, on the other hand, it is necessary to heat the mold in a rapid and controlled manner to achieve acceptable efficiency levels. If the techniques employed for cooling (or heating) of the molding apparatus are too slow, the molding equipment must be slowed down to a speed that is economically undesirable. On the other hand, excessively rapid thermal changes in the mold may lead to poor quality in the molded articles.
In those molding processes in which the molded article must be cooled in the mold prior to removal, it has been conventional practice to provide a series of passages for passing a cooling liquid through the mold, at least some of the cooling passages being located quite close to the mold cavity or cavities. Chilled water or other cooling liquid circulated through these mold passages cools the mold and the molded article so that the article can be removed from the mold without distortion. This procedure is inherently somewhat inefficient because a large volume of cooling liquid must be circulated through the mold to achieve the desired cooling. This conventional procedure is also rather difficult to control so as to achieve a maximum cooling rate without cooling the mold too much, which may interfere with efficient operation of the mold in the next cycle. Moreover, the conventional cooling technique is inherently non-selective; most of the mold is cooled, but it is far more desirable to concentrate cooling in that portion of the mold immediately adjacent to the mold cavity. In those processes that require heating of a mold the same basic techniques are conventionally employed, except that the circulated fluid is a hot liquid. The same problems are presented.